Question

Which of the following is/are correct regarding thermal stability of halogen halides?

• A. Thermal stability order: $HF>HCl$
• B. Thermal stability order: $HCl>HBr$
• C. Thermal stability order: $HBr>HCl$
• D. Thermal stability order: $HI>HBr$

Answer 1

Answer: (A), (B)

Thermal stability order: $HCl>HBr$

On moving down the group, the bond dissociation enthalpy $\left({\Delta }_{diss}H\right)$ of $H-X$ bond (where $X=F,Cl,Br,I$) decreases and hence thermal stability also decreases. Therefore, the correct order of thermal stability of hydrogen halides is:
$HF>HCl>HBr>HI$

Molecule	${\Delta }_{diss}H$ (in kJ/mol)
$HF$	$574$
$HCl$	$432$
$HBr$	$363$
$HI$	$295$